Electrolysis of sodium sulfate



Patented Feb. 16, 1954 ELECTROLYSIS F SODIUM SULFATE William P. Dooley, Dunbar, W. Va., assignor to American Viscose Corporation,

Wilmington,

Del., a corporation of Delaware Application March 30, 1950, Serial N 0. 152,860

2 Claims.

This invention relates to a method for electrolyzing aqueous solutions of salts and to a novel electrolytic cell for effecting the electrolysis. In particular, the invention is directed to a method of and novel electrolytic cell for electrolyzing solutions of alkali metal salts to recover the alkali metal hydroxide and acid therefrom.

One object of the invention is to provide a method and electrolytic cell for electrolyzing aqueous solutions of salts. Another object is to provide an electrolytic cell in which the electrical resistance is maintained at a minimum with consequent increase in the density of the current acting upon the electrolyte.

In accordance with the invention, the solutions are electrolytically decomposed in an electrolytic cell provided with special vertical electrodes. The cathode is constituted by a moving stream of mercury or preferably of an amalgam of mercury with copper or silver, which latter increase the electrical conductivity. The anode is constituted by a moving stream of mercury or an amalgam of mercury with bismuth, antimony or lead which form an insoluble salt with the ions liberated at the anode.

When the electrolyte is electrolytically decomposed in a cell, the electrodes of which are of the above-mentioned type, the cations are neutralized with the production of an amalgam with mercury, which, in accordance with this invention, is continuously removed from the vicinity of the cathode, and the anions are neutralized with the production of a salt which is insoluble in the electrolyte, precipitated in the cell, and continuously withdrawn from the vicinity of the anode. The withdrawal of the neutralized anions and cations as soon as they are neutralized permits the use of a higher current density than would otherwise be possible which reduces the amount of equipment needed to electrolyze a given weight of any particular electrolyte. Also, the fact that an excessive distance between the electrodes, which raises the electrical resistance in the cell, is not required enhances the efiiciency of the cell. Since the products formed at the electrodes are insoluble under the conditions existing in the cell (the voltage impressed on the electrode being greater than the minimum required to prevent the sodium or other metal in the amalgam from reacting with water) and are promptly, if not almost immediately, withdrawn from the cell, the relatively non-porous diaphragm usually required for separating the electrodes is not necessary'and-may be dispensed with altogether, permitting close spacing of the electrodes. The electrodes may be separated by a distance of only A" if desired. Where it is preferred for some special reason to interpose a diaphragm between the electrodes, a highly porous diaphragm which does not appreciably increase the overall electrical resistance in the cell may be used.

The mercury or mercury amalgam constituting the anode and cathode may flow over the surfaces of metal plates or the like which are removably suspended in the cell. Preferably, however, hollow anode and cathode supports of a perforate material, such as a fine-mesh screenlike material, formed as a porous receptacle or container, are provided, and the anode and cathode fluids pass through the perforate supports from the inside to the outside thereof. The cathode and anode supports are formed of a material which is chemically resistant to the electrolyte and the products formed at the electrodes. In order to equalize distribution of the fluids over the outer surface of the supports, the fineness of the mesh of the material may increase from the top to the bottom of the supports and/or additional layers of the screen-like material may be added to the supports as the depth thereof increases from top to bottom. The products resulting from neutralization of the anions and cations, which are substantially insoluble in the electrolyte settle at the bottom of the cell and are continuously or intermittently withdrawn therefrom.

In the drawing, which is illustrative of the invention,

Figure 1 is a sectional elevation of an electrolytic cell constructed in accordance with the preferred modification of the invention,

Figure 2 is a plan view of the cell of Figure 1,

Figure 3 is a sectional elevation of another modification of the invention,

Figure 4 is a sectional elevation of another modification of the invention, and

Figure 5 is a flow sheet illustrating the invention as embodied in a continuous cyclic electrolytic process.

As previously indicated, aqueous solutions of any salt may be electrolyzed in accordance with the invention. However, the invention will be described in detail in connection with the electrolytic decomposition of an aqueous solution of sodium sulfate with the production of sodium hydroxide.

Referring to the drawing, there is shown a vessel 2 whichmay be cast or stamped or other- Wise fabricated from iron or other metal or from glass, rubber, wood, or any other desired nonmetallic material. When the vessel is made of a material which conducts electricity and/or is subject to corrosion by the materials to be electrolyzed, such as metal, it may be provided with a lining of non-conductive, corrosion-resistant material, such as ebonite, celluloid, glass or a synthetic resin, such as polyvinyl chloride, polymers of trifluorochloroethylene, polymers of tetrafiuoroethylene, etc. The configuration of container or vessel 2 is such as to provide a tank or cell in which the electrolysis may be accomplished and to provide a pair of recesses or collecting basins 3 and 4 in the bottom thereof into which the hollow anode and cathode supports 5 and 6 may project.

The cathode support is formed of a fine-mesh material, such as nickel, iron or any other inorganic or organic screen-like material, or a fabric made of yarns of glass, polymers of trifluorochloroethylene, tetrafluoroethylene, etc., which is chemically resistant to mercury or to amalgams of mercury with copper or silver and the electrolytes in the cell. The anode support is also formed of a fine-mesh material, such as any of the materials mentioned for making the cathode support, which is chemically resistant to the insoluble salts formed at the anode which may be chlorides, sulfates or oxides of mercury, bismuth, lead, or antimony as well as the electrolytes present in the cell.

Extending longitudinally of the vessel 2 are rectangular bars 5c and 6a which are provided with openings through which pipes l3 and [4 project. The bars 5a and 6a are provided with depending portions which extend into the container 2. The supports 5 and 6 are removably secured or clamped to the depending portions of 5a and 6a by means of pins or screws, as shown at 50 and 6c. The bars 5a and 6a rest upon pairs of rods '52) and 6b which extend longitudinally of vessel 2 and which are fixed in the end walls of the container 2. Rods 5b and 6b are formed of a suitable insulating material. The supports 5 and the pairs of rods 51) and 6b respectively, so that their flanges are clamped or held between bar 6a and rods 6b, in the case of the cathode support, and between bar 5a and rods 51), in the case of the anode.

The recesses 3 and 4 are provided with outlets 1 and 8, having connections with vessels 9 and ID in which the insoluble products withdrawn from the cell, as by means of pumps II and I2, are collected.

In the operation of the cell, the electrolyte is introduced into the cell as through a pipe I5, and may consist of a saturated solution of sodium sulfate or may be a waste coagulating bath of the viscose rayon industry containing 8 to 15% sulfuric acid, 15 to 33 sodium sulfate and 0.5 to 5% of zinc sulfate, magnesium sulfate and/or aluminum sulfate, previously filtered and freed of sulfur by-products, including CS2 and H23, at a temperature of from about 40 to 60 C. When operating upon a viscose coagulating bath, diaphragms are preferably used, and its rate of passage through the cell may be, and preferably is, timed to convert only a portion of the sodium sulfate and the electrolyte may simultaneously be subjected to partial vacuum evaporation to thereby discharge from the cell a solution of sulfuric acid and sodium sulfate concentrations which correspond to those desired in the viscose coagulation and regeneration-process. The v'acu- 6 which are flanged at the top fit between um should not be high enough to cause boiling or any considerable bubble evolution or prevent the removal of the mercury streams from recesses 3 and 4. The cathode fluid, which may be mercury or a copper and/or silver amalgam is fed into the hollow cathode support 6 through pipe l3 and is maintained at a constant level in the support by a suitable flow control device (not shown) while the anode fluid, which may be mercury or a bismuth, antimony and/or lead amalgam, is fed into the hollow anode support '5 through pipe 14, being also maintained at a constant level in the support. The cathode and anode fluids emerge through the interstices of the fine mesh material constituting the cathode and anode supports and trickle or flow downwardly over'the outer surfaces of the respective supports in the form of a thin film. The electrolyte introduced through pipe l5 and passing upwardly through the cell is electrolyzed and reduced at the cathode to form an amalgam of sodium with mercury, while at the anode an insoluble sulfate, such as that of mercury, bismuth, or antimony or mixture thereof is formed and precipitated. The products settle in recesses 3 and 4 and are withdrawn to vessels 9 and [0 at a constant rate, or the products may be withdrawn intermittently, if desired.

The sodium amalgam collected in vessel 9 is washed with water, for example, in a packed tower or in any other suitable manner known to the art, such as in the conventional amalgam electrolytic decomposer cell, and decomposed in contact with suitable electrodes to NaOH, Hg and Hz, the mercury being recovered for reuse as cathode fluid either per se or as an amalgam with copper or silver, and the aqueous sodium hydroxide being forwarded to a suitable storage tank or the like. If desired, sodium sulfide may be produced instead of sodium hydroxide by introducing NazSc or higher sodium polysulfldes in the water of the electrolytic decomposure cell. The reactions are:

Part of the NazS obtained may be reacted with sulfur to produce NazSz and reused in the cell.

When zinc or other salt, such as those of magnesium, aluminum or like metal, is present in the electrolyte, part of such metal is deposited on the mercury cathode along with the sodium to form a ternary amalgam, such as sodium zinc mercury. This amalgam can be subjected to the normal electrolytic decomposition in water between the conventional electrodes with the production of sodium 'zincate. This can then be precipitated as zinc hydroxide by adjustment of the alkalinity. This amalgam can also be purified by distilling off the mercury, or by oxidation of the zinc by agitating the amalgam with air, followed by removal of the zinc oxide scum, as by skimming off or filtration. Or the amalgam may be treated with an acid, such as sulfuric acid, which dissolves the zinc but not the mercury.

The precipitated sulfate is withdrawn from recess 3 to vessel 9, is calcined with the production of an oxide, such as of Bi, Sb, or Pb, and liberation of sulfur dioxide or trioxide or a mixture thereof, which may be converted to sulfurous or sulfuric acid or mixtures thereof in any known way, as by absorption in a stream of dilute sulfurous or sulfuric acid. If desired, the hydrogen liberated when the Sodium amalgam formed at the cathode is decomposed by water in the decomposer section of the cell processmay be used to reduce the oxide, such as that of bismuth to the metal, such as bismuth, the latter being again amalgamated with mercury and returned to the anode support.

The spent liquor leaving the cell through overflow 12a may be brought back to the saturated condition by the addition of anhydrous sodiumsulfate or Glaubers salt and then returned for re-introduction into the cell through pipe 15.

Alternatively, the cell may be operated under partial'vacuum, part of the excess water of crystallization of Glaubers salt being removed within the cell.

The invention is not limited to the electrolysis of aqueous solutions of the salts, as solutions thereof in other solvents may beelectrolytically decomposed in a cell of the type described. For example, when a solution of sodium sulfate in liquid sulfur trioxide was passed through a cell provided with a cathode constituted by a moving stream of copper amalgam and an anode constituted by a moving stream of mercury, an insoluble sodium amalgam was formed at the oathode and insoluble mercuric sulfate was formed at the anode and precipitated in the cell. The insoluble amalgam produced at the cathode was worked up for the recovery of sodium hydroxide,

and sulfuric acid was obtained by processing the insoluble mercuric sulfate in a manner similar to that previously described for bismuth sulfate.

In the modification shown in Fig. 3, the anode and cathode supports 5 and 8 are secured to the outside of hollow perforate supporting members l6 and H, by pins or rivets i8. Supporting members it and ii are preferably less highly perforated than the supports 5 and 6. The anode and cathode fluids are introduced into the inner supporting members i8 and I1, and flow through the perforations therein and through the interstices of the supports 5 and 6. The perforate members it and l! furnish reinforcement for the fine-mesh supports 5 and 6.

A plurality of cells of the type described may be connected in series, if desired, the individual cells being suitably insulated from one another. Also, a plurality of alternately arranged anodes and cathodes may be disposed in a single cell, as shown in Fig. 4.

Referring to Fig. 4, there is shown a vessel 19 in which are supported, such as by cross pieces 5&- which are suitably vented, as at 51, a plurality of alternately disposed anodes and cathodes 2d and El constructed similarly to the anodes and cathodes illustrated in Fig. 1. Each of the anodes and cathodes projects into recesses 22 and 220 provided in the bottom of the vessel. Anode fluid is introduced into the anode supports through a manifold pipe 23 having leads to each of the anode supports and cathode fluid is introduced into the cathode supports through a manifold pipe it having leads to each of the oathode supports. The insoluble salt formed at the anode and collected in recesses 22 are withdrawn therefrom through conduits 25, the amalgam formed at the cathodes being withdrawn through conduits 2d. The electrolyte is introduced into vessel it through one or more inlets 2'! spaced horizontally at the level shown, the spent liquor leaving the member through one or more outlets 28 spaced horizontally at the level shown. This arrangement of alternately disposed anodes and cathodes in a single vessel is made possible ports 30 and 3| and 33. The cathode may be constituted by a by virtue of the fact that the products formed at both the anode and cathode are immediately withdrawn from the vicinity of the electrodes.

In accordance with the invention, the electrolytic decomposition may be carried-out as a batch procedure, or on a continuous cyclic basis. The flow sheet constituted by Fig. 5 is illustrative of a cyclic process for the electrolytic decomposition of an aqueous solution of sodium sulfate, the invention being described in terms of sodium sulfate solution merely by way of exemplification, it being understood that solutions of other salts may be electrolytically decomposed in a cell constructed in accordance with the invention, if desired. Referring to Fig. 5, the cell comprises a vessel 29 and anode and cathode supwhich project into recesses 32 copper amalgam contained in the hollow cathode support and flowing downwardly over the outer surface of the support. lf'he anode may beconstituted by a bismuth amalgam contained in the hollow anode support and flowing downwardly over the outer surface of the support. The sodium amalgam formed at the cathode is withdrawn from recess 33, as by continuous operation of pump 34, to vessel 35, from which it is pumped,

by means of a pump 36, to a washing tower 31 in which the packing is preferably charged positively to facilitate the decomposition of the sodium amalgam. The water enters at 31c and the aqueous sodium hydroxide is led off at 37b, and the mercury is recirculated, through conduit 38 to hollow cathode support 3|. The bismuth sulfate formed at the anode is withdrawn from recess 32, by continuous operation of pump 39 to vessel 40, from which it is forwarded to a roaster 4|. In the roaster bismuth oxide is produced, with liberation of sulfur oxides. The bismuth oxide is next sent to a heater 42, where it is reduced to bismuth, designated M in the drawing. The metal is then introduced into a heater 43, where it is amalgamated with mercury, designated M, the amalgam MM being then recirculated to the hollow anode support 30 through conduit 4d. The spent liquor leaving the tank 29 is preferably brought back to saturated condition by the addition of anhydrous Na2SO4 or Glaubers salt thereto, in vessel 45, ald is recirculated to tank 29 through conduit 4 The electrolyte entering the cell may be heated or cooled, and/or external heat may be applied to the cell, if desired. When the electrolysis is carried out on a continuous scale, the temperature may be controlled by the continuous flow of the electrolyte and evaporation, when vacuum is used, thereby preventing overheating of the solution at the high current densities used.

Although the invention contemplates the use of anodes constituted by a moving stream of a lead amalgam, in some instances when the electrolyte consists of a solution of sodium sulfate, it is preferred not to use a lead amalgam since lead yields cations which migrate to the oathode, are neutralized, and react with the sodium to form plumbate anions, which is undesirable when sodium hydroxide of a high degree of purity, such as is suitable for use in the preparation of viscose, for example, is to be obtained.

The invention provides a method which may be operated on a batch or continuous basis for the electrolytic decomposition of solutions of salts under conditions such that the electrolyte is acted upon by a current of high density which.

prising an however, requires only a low voltage, so that the production of the desired end products rendered highly efiicient and economical.

I claim:

1. In the electrolysis of an electrolyte comprising an aqueous solution of sodium sulfate, the improvement which comprises subjecting the electrolyte to electrolysis in contact with a cathode constituted by a downwardly moving stream of an amalgam of mercury with copper, whereby the sodium ions liberated at the cathode are neutralized and form an amalgam with the stream, and with an anode constituted by a stream of a substance selected from the group consisting of mercury, lead amalgam, bismuth amalgam, and antimony amalgam, which form insoluble metal sulfates with the sulfate ions liberated at the anode moving downwardly in parallel relation to the cathode, whereby the sulfate ions are neutralized and precipitated in the form of an insoluble metal sulfate, and continuously withdrawing the neutralized sodium ions from the vicinity of the cathode in the form of a sodium-mercury amalgam, while simultaneously continuously withdrawing the precipitated metal sulfate from the vicinity ofthe anode.

2. In the electrolysis of an electrolyte comaqueous solution of sodium sulfate, the improvement which comprises subjecting the electrolyte to electrolysis in contact with a cathode constituted by a downwardly moving stream of an amalgam of mercury with copper, whereby the sodium ions liberated at the cathode are neutralized and form an amalgam with the stream, and with an anode constituted by a stream of an amalgam of mercury with bismuth, moving downwardly in parallel relation to the cathode, whereby the sulfate ions liberated at the anode are neutralized and precipitated in the form of insoluble bismuth sulfate, and continuously withdrawing the sodium mercury amalgam from the vicinity of the cathode, while simultaneously continuously withdrawing the precipitated bismuth sulfate from the vicinity of the anode.

WILLIAM P. DOOLEY.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 651,396 Street June 12, 1900 678,816 Shinn July 16, 1901 942,207 Kitsee Dec. 7, 1909 1,970,974 Palmaer et al Aug. 21, 1934 2,230,023 Aten Jan. 28, 1941 2,273,036 Heise et al. Feb. 17, 1942 FOREIGN PATENTS Number Country Date 490,911 Great Britain Aug. 23, 1938 

1. IN THE ELECTROLYSIS OF AN ELECTROLYTE COMPRISING AN AQUEOUS SOLUTION OF SODIUM SULFATE, THE IMPROVEMENT WHICH COMPRISES SUBJECTING THE ELECTROLYTE TO ELECTROLYSIS IN CONTACT WITH A CATHODE CONSTITUTED BY A DOWNWARDLY MOVING STREAM OF AN AMALGAM OF MERCURY WITH COPPER, WHEREBY THE SODIUM IONS LIBERATED AT THE CATHODE ARE NEUTRALIZED AND FORM AN AMALGAM WITH THE STREAM, AND WITH AN ANODE CONSTITUTED BY A STREAM OF A SUBSTANCE SELECTED FROM THE GROUP CONSISTING OF MERCURY, LEAD AMALGAM, BISMUTH AMALGAM, AND ANTIMONY AMALGAM, WHICH FORM INSOLUBLE METAL SULFATES WITH THE SULFATE IONS LIBERATED AT THE ANODE MOVING DOWNWARDLY IN PARALLEL RELATION TO THE CATHODE, WHEREBY THE SULFATE IONS ARE NEUTRALIZED AND PRECIPITATED IN THE FORM OF AN INSOLUBLE METAL SULFATE, AND CONTINUOUSLY WITHDRAWING THE NEUTRALIZED SODIUM IONS FROM THE VICINITY OF THE CATHODE IN THE FORM OF A SODIUM-MERCURY AMALGAM, WHILE SIMULTANEOUSLY CONTINUOUSLY WITHDRAWING THE PRECIPITATED METAL SULFATE FROM THE VICINITY OF THE ANODE. 